1. Field of the Invention
The present invention relates to a process for the production of low benzene content gasoline. More particularly the invention relates to a process wherein a full boiling range naphtha is fractionated to separate out a light naphtha fraction, a medium naphtha fraction containing the benzene and a heavy naphtha. More particularly the invention relates to a process wherein the medium naphtha is hydrogenated to convert the benzene to cyclohexane.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling range which determines the composition. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials, aromatic compounds and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the “sweet” (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (=⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations. Some users require the sulfur of the final product to be below 50 wppm. In addition the EPA requires that the benzene content of the gasoline be low, i.e., 1 vol. %.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:RSH+H2→RH+H2S  (1)RCl+H2→RH+HCl  (2)2RN+4H2→2RH+2NH3  (3)ROOH+2H2→RH+2H2O  (4)
Typical operating conditions for the HDS reactions are:
Temperature, ° F.600-780Pressure, psig 600-3000H2 recycle rate, SCF/bbl1500-3000Fresh H2 makeup, SCF/bbl 700-1000After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha. The loss of olefins by incidental hydrogenation is detrimental by the reduction of the octane rating of the naphtha and the reduction in the pool of olefins for other uses.
Generally refiners tend to prevent benzene from entering the gasoline blending stock. For example as mentioned above the cracked naphthas may be subjected to aromatic removal by solvent extraction. This, however, removes all aromatic material not just the benzene. One method of preventing the introduction of benzene into the gasoline pool is to remove the benzene precursor (isohexane) from the charge to the catalytic reforming units. This does not solve the problem of streams which contain benzene as well as heavier aromatic compounds such as toluene and xylenes. The heavier aromatics contribute greatly to the octane pool and to date have not been found to be detrimental to the environment.
U.S. Pat. No. 5,7734,670 discloses a process for the hydrogenation of aromatics in a petroleum stream. However, like solvent extraction, the process is not selective to only the benzene. U.S. Pat. No. 5,856,602, discloses the hydrogenation of aromatics in a hydrocarbon stream utilizing a distillation column reactor wherein the placement of the catalyst bed and operation of the distillation column controls which aromatic is retained in the catalyst bed for hydrogenation. U.S. Pat. No. 6,187,980 B1 discloses a process for the hydrogenation of benzene to cyclohexane in a distillation column reactor wherein essentially pure benzene is used as the feed to the reactor.
In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherification. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and causing a loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 250° F. and a heavy boiling range naphtha which boils in the range of from about 250-450° F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. In the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
U.S. Pat. No. 5,510,568 discloses a process in which naphtha is fed to a distillation column reactor which acts as a depentanizer or dehexanizer with the lighter material containing most of the olefins and mercaptans being boiled up into a distillation reaction zone where the mercaptans are reacted with diolefins to form sulfides which are removed in the bottoms along with any higher boiling sulfur compounds.